pulling 12v without a batt imbalance

[trukhead]

So flat out don't use rear For Any!!! 12v because of possible unforeseen breakdown in alternator, don't be dependent on something that can fail and cause further and more severe damage, Is that it Doghead?, Thanks and I just want to understand more as I travel down this road of CUCV

Think of it like this. If you take a positive lead and a negative lead(pair of jumper cables) off of the rear battery(next to the firewall) and stand away from the truck, you will be at least 12 volts(negative lead) higher than the fender you are leaning on. Now you tap the positive cable(positive lead) against the fender and you get a 24 volt ZAP!!
The rear battery( and its alternator) is in series to the front battery.
You can tap off the rear battery but most everything that uses voltage has a chassis ground.
So having an electrical do-dad getting powered off of the rear battery and having it touch the truck at any point is like bouncing the negative or positive jumper cables off of the fender in the top example.
You have to have whatever electrical do-dad you use (that gets voltage from the rear battery) floating in space (air).
Every thing powered off of the rear battery is at least 12 volts potential to as much as 24volts relative to the truck surfaces. It will make a ZAP if it contacts the truck surfaces!opcorn:

 

[Carlo]

The Cucv escapes normal thinking in the realm of adding 12V DC electronics to a 24V system and keeping batteries topped off and balanced. Most of us think of a Deuce or 5 ton where the Alt or Gen send 24V power to one battery wired in series to the "trailing" battery. I agree with what was posted earlier, do just as GM did, install an ungrounded terminal block connected to the rear batteries negative post and run the accessories from the rear batteries positive post. The front battery supplies positive power to your mostly 12V truck and the factory run 12V components and it's alternator keep it charged. You're rear battery is serviced by an ungrounded 12V alternator, provides the power to you're accessories if you add some, and it's dedicated alternator keeps it charged. Now keep in mind, said accessories will be toast if the insulating shims in the passenger side alternator fail. You could pull straight from the fuse panel if you feel your electrical needs will be under the amperage draw and the driver's side alt could keep up. If you wire it this way, keep your eyes peeled for a constant Gen 1 light, that indicates you've overdrawn that circuit and need to re-wire as I've described.

Very well said.

 

[allenhillview]

Think of it like this. If you take a positive lead and a negative lead(pair of jumper cables) off of the rear battery(next to the firewall) and stand away from the truck, you will be at least 12 volts(negative lead) higher than the fender you are leaning on. Now you tap the positive cable(positive lead) against the fender and you get a 24 volt ZAP!!
The rear battery( and its alternator) is in series to the front battery.
You can tap off the rear battery but most everything that uses voltage has a chassis ground.
So having an electrical do-dad getting powered off of the rear battery and having it touch the truck at any point is like bouncing the negative or positive jumper cables off of the fender in the top example.
You have to have whatever electrical do-dad you use (that gets voltage from the rear battery) floating in space (air).
Every thing powered off of the rear battery is at least 12 volts potential to as much as 24volts relative to the truck surfaces. It will make a ZAP if it contacts the truck surfaces!opcorn:

So do I not ground my device or do I ground and not let( it) touch, I do see the terminal block Gm used so did they not ground anything else after leaving the block with 12v through-out the rest of the truck?

Also (sandcobra) I,m trying to accomplish what you said in post #22 just understand it more, Do I use ground on device (acc.) or not? I know there is 12v on rear battery but I don't understand the ground issue, I do electrical stuff alot around here but don't see this one as its being explained in above post.

GM used the 12v for acc what am I not understanding?

 

[doghead]

Show us a picture of the amp. Show us the schematic. Tell us the maximum load.

Think about this, if you wire it to the rear battery, and mount it isolated from the body/frame, and something conductive touches it and the body(metal), you will have a dead short (assuming it is chassis grounded).

Use the front battery only, for 12 volt loads. If that alternator does not have enough capacity for the additional load, replace it with a larger capacity alternator.

You need to provide more facts.

 

[Cleveland Iron]

I hate to say it... But there is such a thing as a DC autotransformer. ( I can only guess what sort of electronic magic they use to make them work. ) They are not very well known but they are popular in the solar industry.

The DC autotransformer is a bidirectional device solid state device that mimics a three terminal AC autotransformer. I use a 12v - 24v unit on my deuce as a battery equalizer. It also allows me to charge both batteries with a 12V charger if I need to. They aren't cheap but they work just fine.

Here is a a sample list of these units.

http://www.aeesolar.com/catalog/solar-converters-dc-autotransformer

 

[MarcusOReallyus]

I hate to say it... But there is such a thing as a DC autotransformer.

No there isn't!


There are only ignorant manufacturers using the wrong term for a converter.


It's a converter, not a transformer.

 

[3dAngus]

This is leading into an argument based on semantics now.
What that really is, is called a converter. It converts one form of DC to another.

The way it does it, is it "essentially" converts DC to a form of AC by chopping the DC, and making a square wave out of it. You take the square wave, which is also AC just like a sine wave is, and can convert it to a higher voltage, or lower, your choice depending on how you wind the transformer.

It's AC at this point.

After you select the right voltage conversion level through the tapping of the transformer windings, you rectify it again and get DC at your preferred voltage.

It's a converter.

It uses a transformer because it converts DC to AC first via the chopped square wave.

You're both right, but the correct nomenclature for that device is a converter, rather then autotransformer. Autotransformer is more of a marketing term, rather then a scientific nomenclature.
It is becoming more common to use it, but that doesn't make it right.

 

[doghead]

Semantics only confuse the situation.

 

[MarcusOReallyus]

So now I'm confused bad, there is 12 v on rear battery right?,

No No No No No No No No No No No No!



Yes.

Maybe.

It depends...


Confused even more? Good!


Here's the part that confuses people - there's no such thing as 12v at some point. OR 24v, or 120, or any volts.

Voltage is a measure of the difference in electrical potential between two points.


Unless you know which two points you are talking about, it's meaningless to talk about 12v or 24v or whatever volts. In fact, it can be dangerous.


So, have you got 12v at the rear battery positive terminal? No. Yes. Depends.


12v compared to WHAT? What's the other reference point?


The negative terminal of the rear battery? Yeah, there's 12v between the positive terminal and the negative terminal of the battery. Fine.

But what about the vehicle's ground?

Ooops. Measure from the vehicle's ground to the positive terminal of the rear battery and you'll see TWENTY FOUR volts.


So, can you hook a radio to the rear battery?

Well, yes. And if you connect the power lead of that radio to the positive terminal of the rear battery, and then you ground that radio, that poor radio is going to get TWENTY FOUR volts.

Smoke!

Now, if you are very careful to make sure that NO PART of the radio EVER touches ground, you can run the ground lead of the radio to the negative terminal of the rear battery and it will work just fine.

Well.... until one day when something goes wrong and that radio's case or antenna lead or something touches vehicle ground, and now you'll be sending out smoke signals from somewhere.



This is the critical thing to learn about electricity - there's no such thing as 12 volts, unless you know the other measuring point. "12v" by itself means nothing.


We talk that way with conventional automotive systems because it's assumed that vehicle ground is the reference point. And that's fine in an ordinary 12v system.

But once we introduce a mixed 12v/24v system, it's critical to understand that you must be clear on the reference point.

 

[MarcusOReallyus]

Semantics only confuse the situation.

Using the correct nomenclature corrects the confusion.

 

[3dAngus]

Good Grief!

Why does everything have to turn into a

"I'm smarter then you are" type of argument. Once again, this has just become ridiculous!

Maybe we should just stay on track and answer his question. I wish I knew more about CUCVs but I've never been in one.

I would refer all to the post submitted by Sandcobra as the best one with any intelligence in it for an answer. And yes, determine the load. Nuff said.

12v is not 12v? Phooey!

 

[MarcusOReallyus]

Hey, I'm just trying to help him understand how this works. A degree in the stuff and ~30 years of working with it have taught me enough about it that I do expert witness work (criminal cases, usually) on the side of my day job as an engineer. My work is cited by MIT as "suggested reading" for their Technology MBA degree. I think that qualifies me to try to help other people understand it.

12v is not 12v volts, unless you know the reference point. If that confuses you, join the club. It's confusing to a lot of people. It was to me before I went to school to learn about it.

It's still true.

 

[trukhead]

So do I not ground my device or do I ground and not let( it) touch, I do see the terminal block Gm used so did they not ground anything else after leaving the block with 12v through-out the rest of the truck?

...GM used the 12v for acc what am I not understanding?

You will have to complete a circuit to make the electrical do-dad work. BUT YOU CAN not ground anything to any surface of the truck!! If you use the rear battery for 12 volts to power something, you must ground it back to the rear battery and no where else! For example, if you mount a stereo in your truck and it is completely isolated from the cab, frame etc... you will need to run a positive and negative wire to the battery but remember this,

WHEN YOU HOOK UP THE ANTENNA YOU WILL NEED TO ISOLATE IT AS WELL AND RUN A GROUND WIRE TO THE BATTERY AS WELL! THE ANTENNA will ALSO GROUND THROUGH ITS BASE.

The 12 volts from the rear battery exist only in our perception. From the point of view of the truck, it is the last half of 24 volts. The truck sees the two batteries as one battery with one ground(the chassis of the truck). GM is sneaking 12 volts out of the first half of the battery where that 12 volts can return to the negative via the front ground(chassis), the 12 volts that GM snuck out is the ground for the last half of the 24 volt battery.

The ground on the rear battery is actually coming from the middle of the "battery" ( two 12 volt batteries in series) from the trucks point of view.

Using 12 volts from the rear battery is like being in a bubble and it needs to stay in that bubble.

Think of it like getting into on of those bubbles you can ride in down a ski slope. The ski slope is the isolated environment that grounding danger does not exist. Now you are in your bubble going down the slope in your isolated environment, everything is grand. Now your bubble goes off course and touches a boundary and your bubble leaves the safe isolated environment and you touch the boundaries and bad things happen and over the cliff you gooooooo........


Do not use the negative terminal block on the firewall where the big wires go.

The 12 volt junction up by the glow plug relay would work as a ground for the rear battery 12 volts but i would only use it the event of an emergency.

Get your self a high quality basic Fluke VOM. Learn how to use it on the amps, ohms and ac and dc volts and go play under the hood.

Jump between the rear battery negative terminal and the 12 volt junction by the glow plug relay (solenoid).

When you think it is safe to ground, check it with the fluke and see if you have voltage

 

[MarcusOReallyus]

Using 12 volts from the rear battery is like being in a bubble and it needs to stay in that bubble.

That's a good way to put it!

 

[doghead]

Do not use the rear battery.

Post more information when you have it.

Everyone else, STOP.

 

[Skinny]

I would just make all of my aux lighting 24v. Even if you can't find them, get a 12v housing and put 24v bulbs in them...convert the headlamps as well using a Hella relay kit off the 24v bus bar. As far as stereo amps go...I'm sure some Marine or RV application may have something out there but good luck finding that. If you really need to run that many high draw 12v devices, why not just convert to 12v and use both alternators and batteries in parallel. This would better suite your application in my opinion. You will lose out on your 24v starting circuit which is the best thing about these trucks. Gotta make a decision here.

 

[doghead]

He can replace the D/S alt to about a 200 amp alt.


There's no point in this until he knows what he needs.